Draft: Port to Gtk4 [take 2]

Supersedes: !129 (closed)

There are some differences with the gtk3 version, but most of them seem to be details to me.

In particular dealing with display scaling isn't well thought. I just left a simple "scaling" switch option. The good news is that improving the situation in RDW should help all implementations (RDP/VNC/Spice/QEMU).

Otherwise, the port should be good/equivalent from a regular user level.